1. Introduction
This invention is directed to improved phosphating compositions for ferrous based metals.
2. Description of the Prior Art
As is well known in the art, phosphate coatings are conversion coatings for iron based metals. The coatings serve as a base for organic coatings to aid in cold forming, to improve wear resistance or to impart color and to provide corrosion resistance to the base metal. The coatings are for the most part phosphates of metals in the phosphating solution (the primary metal) and of iron from the base metal. Formation of a phosphate coating is by contact of the base metal with a phosphating composition for a time and at a temperature necessary to provide a coating of the desired thickness. Methods and compositions for phosphating are well known and disclosed in numerous publications including, for example, The 44th Annual Edition of Metal Finishing Guidebook and Directory, Metal and Plastics Publications, Inc., Hackensack, N.J., 1976, pp 554-566; Burns and Bradley, Protective Coatings for Metals, Reinhold, 1967, 3rd Edition, pp 568-575; and U.S. Pat. Nos. 2,164,042; 2,326,309; and 2,351,605, all incorporated herein by reference.
Compositions for phosphating a surface typically comprise a dilute aqueous acidic solution of a metal phosphate formed by the dissolution of a primary metal salt in phosphoric acid, free salt, phosphoric acid and an oxidizing agent as an accelerator. The metal salt dissolved in the phosphoric acid is most often zinc oxide with the formation of a primary zinc phosphate but salts of manganese and iron are often used either alone or in combination with the zinc oxide. A phosphate coating is formed by free phosphoric acid attacking the metal surface liberating iron which goes into solution thus providing iron phosphate in solution in addition to the primary metal phosphates. At the interface of a base metal surface and the solution, the pH is altered resulting in the insolubilization of the phosphates and precipitation of the same on the surface of the base metal forming the conversion coating. An overall reaction for formation of the coating can be written as follows using zinc as illustrative of the primary metal in the solution: EQU 3Zn(H.sub.2 PO.sub.4).sub.2 +Fe+4H.sub.2 O.fwdarw.Zn.sub.3 (PO.sub.4).sub.2 4H.sub.2 O+FeHPO.sub.4 +3H.sub.3 PO.sub.4 +H.sub.2
the combination of the zinc and iron phosphates in the above equation represents the phosphate coating.
Though phosphate coatings have been used for many years, one limitation in the use of the conventional phosphating solutions is a sensitivity to contamination by excessive iron phosphates in solution. A fresh phosphating solution is typically free of iron phosphates. Thus, a coating produced therefrom would contain a minimum amount of iron phosphate derived from solubilization of iron by phosphoric acid. As the phosphating solution is used to phosphate additional surface area, the concentration of the dissolved iron in solution increases resulting in a concomitant increase of iron phosphate in the phosphate coating. Some iron phosphate in the coating is beneficial but excessive amounts detract from the quality of the coating. Therefore, when the concentration of iron in solution becomes excessive--i.e., in excess of 0.5% by weight, the coating quality is adversely affected and ultimately, the phosphating solution will no longer be operative.
An additional limitation with most known phosphating solutions is the requirement that they be operated at a relatively high operating temperature, typically in the range of 200.degree. F. This causes difficulty in control of the bath and results in excess energy consumption.
As to phosphate coatings obtained from known phosphating compositions, they improve the corrosion resistance of the surface. For example, typically, using phosphating compositions containing zinc as the primary metal, such coatings are able to withstand about 2 to 8 hours of salt spray in weights of about 2000 to 3000 milligrams per square foot. Decreased salt spray resistance is encountered when manganese is the primary metal. However, greater corrosion resistance is desirable for many purposes.